The ASA investigated after complainants said they had used the site believing it was the official TfL site after it was returned high up in Google searches. It charges £16 to pay the £10 charge and complainants said the site, which used the TfL typeface, colour scheme and a tab labelled "Visit Transport for London", had given the impression that it was part of TfL. Paylondoncongestion.co.uk is one of a number of websites that mimic government pages and charge consumers hefty, unnecessary fees for everything from renewing a passport or driving licence to receiving the free European health card.

It charges £16 to pay the congestion charge in advance, whereas drivers could pay it on the official site for just £10. If you pay for previous days it charges £20 instead of TfL's official charge of £12. It justifies the extra cost by saying it charges a "service fee for checking and processing your application".

In its ruling the ASA said: "The ads must not appear again in their current form. We told paylondoncongestion.co.uk to ensure that their ads made clear that they were not affiliated with TfL, and to specify the amount of their fees at or before the point at which consumers were invited to enter their personal details."

In recent years there has been a huge growth in the number of copycat websites designed to trap unwary consumers into paying fees. In most cases users Google the service they need and then click on one of the Google ad sites, which takes them to the copycat site. Users think they are dealing with the real government site.

It is not against the law for a company to offer a service similar to that of an official government body and to charge for it, but those duped have repeatedly asked why the government has failed to act.

The ASA has commissioned research into the problem, after which it said it would consider taking further action.

Google has insisted that companies have to make it clear they are not the official site.